A portal is a portlet container.
A portal usually provides a security mechanism, a vender specific
implementation,
to control who gets to gets to see portlets, navigation items (client
side menus),
and pages.
One a given page you might have five or six portlets. The portlets
themselves might
be visible separate web sites.
A portal might have several web sites associated with it.
A portlet is a specific application. Like "Search for our US Offices"
could be an
example application for a portlet. "Manage US Offices" would be another
portlet.
However, only people designated as being authorized to see the
management
portlet would see it appear on the page.